The invention relates to a circuit for the generation of a direct control voltage dependent upon an alternating voltage including a charging capacitor which can be charged through at least one charging circuit and can be discharged through at least one discharging circuit which is particularly suited for the generation of a direct control voltage in a system for dynamic compression and expansion.
The German Patent Specification 24 06 258 corresponding to U.S. Pat. No. 3,969,860 issued July 13, 1976 to J. Wermuth has disclosed the use of a circuit of this kind as a control voltage generator in a compander system.
In the disclosed compander system, depending on whether "compression" or "expansion" is desired, to the input of the control voltage generator is supplied a signal derived respectively from the alternating voltage output signal or the alternating voltage input signal of an amplifier located in the useful signal path, the amplification of which is electronically controllable (adjustable) via another electronically controllable amplifier. The direct voltage output signal of the control voltage generator is fed both into the control input of the amplifier located in the useful signal path and also to the control input of the further amplifier. The control voltage generator acts in this context so that on exceeding of a threshold value by the alternating voltage fed into its input, the control voltage generator generates a quickly rising direct voltage which varies the amplification of the amplifier located in the useful signal path until the alternating voltage at the input of the control voltage generator has dropped back below the threshold value.
One example of an embodiment of the prior art control voltage generator disclosed in the patent comprises a charging capacitor, one terminal of which is connected to earth and the other terminal of which is wired, on one hand, through a charging resistance to an operating voltage (supply) and, on the other hand, through a discharge transistor to earth. The alternating potential fed into the transistor base is rectified in the transistor. On transgression of a threshold value by the alternating voltage at the transistor base, the capacitor charged through the resistance is simultaneously relatively swiftly discharged through the instantaneously current conducting switched transistor. In contrast to this very fast discharge of the charge capacitor, its charging is mainly determined by the magnitude of the charging resistance.
It had been found, e.g., in the case of an expander circuit, that two inherently contradictory requirements have to be fulfilled in the described charging of the charging capacitor. The charging should, on one hand, proceed as rapidly as possible so that in the event of a suddenly occurring prolonged decrease in the amplitude of the useful signal requiring to be processed, it is possible to effect a quick correction in the amplification of the amplifier located in the useful signal path. Undesirable noise signals would otherwise become audible in such a case during the long transition time caused by the slow charging. Charging should, on the other hand, not proceed too rapidly to avoid that, in the case of the lowest frequencies to be processed, control is so fast as to produce distortions of the pure sinusoidal oscillations with the lowest frequencies. This problem is particularly manifested in wide band compander systems because, in such systems, all frequency ranges including even the lowest frequencies are processed in the one single available channel.
The above considerations apply correspondingly also, when in another example of an embodiment of the prior art control voltage generator disclosed in the patent, not discharging but charging is effected through a transistor. The above considerations apply in this case to the discharge of the charging capacitor.
A compromise has to be made in the choice of the charge and discharge time constants to meet the aforementioned contradictory requirements.